After a rule of rebound regulations is applied to a golf club head, a method of improving a carry distance is progressing in shaft performance. Most effective means to cover repulsive force of the golf club head is to make a shaft long. A club head speed can be increased by making the shaft long. However, an inertia moment of the club is increased only by simply making the shaft long, so that players may feel the club “heavy” at the time of swing. There is a technique for lightening weight of the club head as a means to solve this problem, but when the weight of the club head is lightened, an impulse is reduced at the time of an impact of the club head with a ball, and thus it is not expected that the carry distance is largely increased. Meanwhile, in the case of lightening the weight of the shaft without changing the weight of the club head, it is possible to reduce only the inertia moment of the club without reducing the impulse at the time of the impact of the club head with the ball. For this reason, a technique for lightening the weight of the shaft has largely received attention.
Patent Document 1 discloses a technique for lightening the weight with paying attention to a bias layer. According to this, in order to improve torsional strength, the bias layer is formed using a material having a thickness of 0.06 mm or less, thereby solving the problem. At this time, a hoop layer is disposed to have two layers in a full length to ensure bending strength. This is because the hoop layer largely contributes to the bending strength.
In Patent Document 2, a length of the hoop layer is disposed to be 20% to 50% of the full length from each of a small-diameter end part and a large-diameter end part of the shaft. As the hoop layer is not present at an intermediate portion, the weight of the shaft is lightened by that much and strength required for shaft characteristics can be ensured at a small-diameter side and a large-diameter side.
A problem in the weight lightening of the golf club shaft is a balance between light weight and strength (three-point bending strength (referred to as SG type three-point bending strength reference in Japan; SG type three-point bending strength test complies with a three-point bending test method prescribed by Consumer Product Safety Association), see FIG. 1). In FIG. 1, a symbol “l” indicates a length of 150 mm in T-90 and a length of 300 mm in T-175, T-525, and B-175. Generally, the bending strength required for the golf club shaft varies depending on positions on a shaft S. Particularly, since shock is applied to a front-end part at the time of the impact, the front-end part requires the largest bending strength. With respect to remaining portions, it is known that an approximately constant value is required from a relation between a rigidity value and the amount of bending. In addition, an individual method or criteria of a strength test is provided by each of club makers, but it is known that it is necessary to satisfy strength reference values of Table 1 in a three-point bending strength test so as to pass such a strength test. That is, a position of T-90 (in the case of the SG type three-point bending strength reference, also referred to as a position T) is a point at which a stress concentration tends to occurs at the time of the impact, a position of T-175 (in the case of the SG type three-point bending strength reference, also referred to as a position A) is a point at which bending deformation tends to increase, a position of T-525 (in the case of the SG type three-point bending strength reference, also referred to as a position B) is a point at which both of a bending load and a crushing load are applied, and a position of B-175 (in the case of the SG type three-point bending strength reference, also referred to as a position C) is a point at which the crushing load is easily applied.
TABLE 1Reference strength standardDesignationT-90T-175T-525B-175Load pointFrom small-From small-From small-From large-positiondiameter enddiameter enddiameter enddiameter end90 mm175 mm525 mm175 mmStrength800400400400reference value[N]
When measuring the strength of a shaft which is prepared using the prior art disclosed in Patent Document 1 described above and satisfies the strength reference, sufficient strength can be obtained at the positions of T-90, T-175, and B-175, but a lowest value is indicated at the position of T-525. This is because the position of T-525 is located approximately in the center of the shaft and the bending load, the crushing load are simultaneously applied as described above, and thus there is a tendency that the strength is lowered compared to the positions of T-90, T-175, and B-175. In the case of using Patent Document 2, the strength at T-525 is further lowered. That is, when the shaft is prepared using the prior art, it is necessary that the strength exceeds a reference value of 400 N (40 kgf) in order to satisfy the reference strength standard even at T-525 having the lowest value. However, in this case, the positions of T-90, T-175, and B-175 (particularly, the positions of T-175 and B-175 to be measured under the same span) becomes an excessive strength state, and surplus weight is distributed to these positions.
Patent Document 3 discloses a configuration where a hoop layer has one layer only at the intermediate portion and the hoop layer has two layers in the full length in order to ensure crushing rigidity of the intermediate portion. However, a position of the hoop layer at the intermediate portion is specified in the range not exceeding 45% of the full length from the large-diameter side (the large-diameter side spaced more than 643 mm apart from the small-diameter side when the full length is 1168 mm). Even when the hoop layer of the intermediate portion is disposed at this position, the strength at T-525 is not improved. This is because an object of Patent Document 3 is a speed-up of return bending rather than the weight lightening.